The invention relates to the field of computers, and more specifically to the control of computer fans.
Most computers or similar electronic devices comprise air cooling systems, with one or more fans. These fans are operated by DC controlled motors. One of the problems encountered with such fans, in office environment, is the problem of the noise of the motor running the fans, and the noise of the fan itself.
Several solutions to this problem have already been proposed.
Variable air cooling for computer and/or electronic equipment, IBM Technical Disclosure Bulletin, vol. 32. no 10A, March 1990, discloses the use of a motor having multiple windings, controlled by a thermistor. The thermistor is mounted near to the air input, and varies the speed of the motor by switching to different winding taps according to the temperature of the ambient air.
EP-A-0 090 514 discusses the drawbacks of a limited number of possible speeds, and discloses a circuit for controlling a DC motor for a fan. This circuit uses a thermistor and an integrated circuit voltage regulator for supplying a variable voltage to the DC motor, according to the temperature detected by the thermistor.
U.S. Pat. No. 5,249,741 discloses a computer system with fans, where the required fan speed is stored in a register; the stored digital value is converted to an analog value for driving a power supply unit. The power supply unit provides to the fan motor a DC voltage representative of the analog value supplied by the digital analog converter. The speed value of a fan is determined at boot based on the cooling requirements of each of the components of the computer system, the cooling requirements being stored in the system. There is no provision in this document for changing the speed value for a fan during operation of the computer system.
U.S. Pat. No. 2,991,405 teaches the use of a thermistor connected to the base of a transistor for controlling the current supplied through the transistor to a motor in a temperature control system.
U.S. Pat. No. 5,687,079 discloses a computer fan control circuit; it suggests controlling the speed of the fan in a computer, according to the temperature of the ambient air detected by a thermistor. The current supplied to the motor operating the fan is controlled by a transistor. The base of the transistor is connected to a circuit comprising a thermistor. A low temperature voltage divider provides a constant low voltage to the motor when air temperature is below 28xc2x0 C. A high temperature voltage divider provides a constant high voltage to the motor when air temperature is above 40xc2x0 C. Thus, voltage supplied to the motor is constant below 28xc2x0 C., increases linearly, and is again constant above 40xc2x0 C.
These prior art circuits suffer a number of drawbacks. Some are not based on common components, and are therefore costly to manufacture. They do not ensure that the fan will start, when operated at the minimal speed.
For a temperature control of the type shown in U.S. Pat. No. 5,687,079, three independent parameters need to be set. The first is the minimal fan speed Vmin. The second is the lower temperature Tmin below which the fan will operate at its minimum speed. The third one is the upper temperature Tmax above which the fan will operate at its maximum speed. The prior art circuits do not allow easy control of these three parameters; however, these parameters need to be adapted to the type of computer system in which the fan is used.
In addition, the maximum voltage supplied to the fan motor should preferably be as close as possible to the maximum available voltage.
Finally, it is useful that the fan motor be driven as accurately as possible.
Therefore, there is a need for a fan motor control circuit providing a solution to one or more of the problems discussed above. The invention thus provides a fan motor control that is easy to manufacture, uses only common components, provides easy setting of the minimal speed, of the lower temperature, and of the upper temperature, ensures that the fan motor will start even when operated at minimal speed. The invention also ensures that the maximum voltage supplied to the motor is close to the maximum available voltage. In addition, the circuit of the invention allows the fan motor to be stopped using a TTL general purpose input/output.
The invention provides a motor control circuit for a computer system fan motor, comprising temperature measuring means outputting a signal representative of the voltage to be applied across fan motor terminals, and feedback control means for applying a voltage across fan motor terminals according to said signal.
In one embodiment of the invention, the circuit further comprises means for stopping the supply of voltage across the fan motor terminals, irrespective of said signal.
In case where the voltage applied to the fan motor varies between a lower voltage and a higher voltage, the circuit preferably comprises means for applying a pulse of the higher voltage to the fan motor terminals when the motor is started, irrespective of the output of the temperature measuring means.
In another embodiment, the circuit further comprises a capacitor in parallel to the fan motor terminals.
The temperature measuring means may comprise a temperature sensor and a sensor amplifier, the temperature sensor comprising a thermistor.
In one embodiment, the sensor amplifier comprises an comparator for comparing the voltage across the thermistor with a reference voltage and for outputting a difference signal representative of the difference between the voltage across the thermistor and the reference voltage, said sensor further comprising means for adding to said difference signal a signal representative of a lower value of said signal.
Preferably, the feedback control means comprise subtracting means, fan control means, and feedback loop means, said subtracting means receiving the signal outputted by said temperature sensing means and a signal outputted by said feedback loop means, and outputting to the fan control means a signal representative of the difference between the signal outputted by said temperature sensing means and the signal outputted by said feedback loop means, said fan control means applying across said fan motor terminals a voltage representative of the signal supplied by the subtracting means, said feedback loop means outputting to said subtracting means a signal representative of the voltage applied to the fan motor terminals.
In this case, the subtracting means preferably comprise an amplifier, the positive input of which receives the signal outputted by said temperature sensing means and the negative input of which receives a signal outputted by said feedback loop means.
The fan control means may comprise a power transistor, the collector of which is connected to one of the fan motor terminals, the emitter of which is connected to one of the ground and power supply, and the base of which is connected to the output of said subtracting means, the other of said fan motor terminals being connected to the other of said ground and power supply.
The means for stopping may then comprise a transistor, the collector of which is connected to the base of said power transistor, the emitter of which is connected to the ground, and the base of which receives a fan stopping signal.
Preferably, the feedback loop means comprise a comparator for comparing the voltage across the fan motor terminals to a reference voltage, and for outputting a signal representative of said difference.
In one embodiment, a highest voltage applied to said feedback control means is less than a highest voltage applied to the fan motor terminals, and the feedback loop means compensate for the difference between said highest voltage applied to the fan motor terminal and said highest voltage applied to said feedback control means.
The invention also relates to a computer including a fan motor connected to such a circuit.